The present invention relates to damping force control type hydraulic shock absorbers attached to suspension systems of automobiles or other vehicles. More particularly, the present invention relates to a damping force control type hydraulic shock absorber capable of appropriately controlling damping force in accordance with the road surface conditions, vehicle running conditions, etc. with a view to improving the ride quality and the steering stability.
In general, a damping force control type hydraulic shock absorber includes a cylinder having a hydraulic fluid sealed therein. A piston connected with a piston rod is slidably fitted in the cylinder. The hydraulic fluid is caused to flow through a hydraulic fluid passage by sliding movement of the piston in the cylinder. The flow of hydraulic fluid is controlled by a damping force generating mechanism formed from orifices and disk valves to generate damping force, and the damping force is controlled by changing the flow path area with a damping force control valve.
The assignee disclosed a damping force control type hydraulic shock absorber in the specification of Japanese Patent Application No. Hei 11-121787. The disclosed damping force control type hydraulic shock absorber has a variable pressure control valve in which a slider valve is slidably fitted in a cylindrical sleeve, and the flow of hydraulic fluid through ports provided in the side wall of the sleeve is controlled in accordance with the position of the slider valve, thereby controlling damping force.
More specifically, an enlarged-diameter portion is formed in the inner wall of the sleeve. A disk valve body supported by the slider valve is seated on a shoulder provided by the enlarged-diameter portion. The plunger of a solenoid pushes the slider valve in the direction for closing the variable pressure control valve, that is, in a direction in which the disk valve body is pressed against the shoulder. The pressure of hydraulic fluid for opening the variable pressure control valve is determined from the relationship between the magnitude of pressing force of the plunger and the pressure of hydraulic fluid applied to the disk valve body.
However, the damping force control type hydraulic shock absorber disclosed in the above-described specification suffers from the following problem. Because each end of the slider valve is disposed in a hydraulic fluid chamber communicating with a hydraulic fluid passage on the downstream side of the variable pressure control valve, the change in the pressure in the hydraulic fluid chambers influences the slider valve. When the flow rate of hydraulic fluid flowing through the variable pressure control valve increases sharply, the pressure in the downstream-side hydraulic fluid chamber rises sharply. This causes imbalance between the pressures in the hydraulic fluid chambers at the two ends of the slider valve. As a result, the slider valve moves undesirably. Accordingly, there is likelihood of damping force becoming unstable.
The present invention was made in view of the above-described circumstances. An object of the present invention is to provide a damping force control type hydraulic shock absorber capable of generating stable damping force even if the flow rate of hydraulic fluid changes sharply.
To attain the above-described object, the present invention provides a damping force control type hydraulic shock absorber including a cylinder having a hydraulic fluid sealed therein. A piston is slidably fitted in the cylinder. A piston rod is connected at one end thereof to the piston. The other end of the piston rod extends to the outside of the cylinder. A hydraulic fluid passage passes the hydraulic fluid in response to sliding movement of the piston. A damping force control valve controls damping force by controlling the flow of hydraulic fluid through the hydraulic fluid passage.
The damping force control valve is a variable pressure control valve having a slider valve slidably fitted in an approximately cylindrical sleeve to control the pressure of hydraulic fluid between upstream and downstream ports provided in the side wall of the sleeve by movement of the slider valve. A hydraulic fluid chamber on the downstream end portion of the slider valve in the sleeve is cut off from the downstream port.
With the above-described arrangement, the pressure of hydraulic fluid between the ports in the sleeve does not act on the hydraulic fluid chamber on the downstream end of the slider valve. Therefore, there is no possibility of the slider valve being moved by the pressure of hydraulic fluid between the ports in the sleeve.
In the damping force control type hydraulic shock absorber according to the present invention having the above-described arrangement, the damping force control valve may be a pilot valve of a pilot operated type damping valve that generates damping force in accordance with sliding movement of the piston.
With the above-described arrangement, the pilot pressure changes as a result of the control of the damping force control valve, causing valve-opening characteristics of the pilot operated type damping valve to change.
The above and other objects, features and advantages of the present invention will become more apparent from the following description of the preferred embodiments thereof, taken in conjunction with the accompanying drawings.